I. Field of the Invention
The present invention relates to the continuous casting of flat metal products, especially flat steel products. More particularly, the invention relates to the management of the flow pattern of the liquid metal poured into the casting mold by employing electromagnetic forces to improve the quality of the cast products and/or the productivity of the casting plant.
It should be noted that the term “flat products” is understood to mean slabs, narrow slabs, thin slabs, etc. or any other product of “elongate” cross section, that is to say one in which the width is at least twice the thickness.
II. Description of Related Art
The molds with which the flat products are cast conventionally have two long sides (or walls) made of copper or copper alloy, which are vigorously cooled by circulating water in contact with them, said long sides facing each other separated by a distance that defines the thickness of the cast product. These full-face walls adjoin their ends by two short side walls so as to form a sealed casting space that reproduces the desired rectangular section. A system for cooling the walls, comprising water chambers and cooling channels, is designed to ensure that sufficient heat is extracted, via these walls, from the cast metal. Sufficient heat extraction results, at the exit of the mold, in the formation of a solidified metal shell in contact with these cooled walls, said shell being uniform around its perimeter and having a thickness of a few centimeters, so as to give the cast product a mechanically strong envelope in order to allow it to completely solidify under traction in the lower stages of the secondary cooling (direct water jets) of the casting machine.
As is known, the free surface of the cast metal in the mold (this surface will be called hereafter the “meniscus”) is generally covered by a covering slag. The metal is therefore poured using a submerged nozzle, which dips a few tens of centimeters below the meniscus in the mold and is provided at its outlet end with lateral outlets via which the liquid metal spurts towards the short sides of the mold.
No one today can ignore the importance of the influence, both on the metallurgical quality (including being inclusion-free) of the cast metal and on the success of the casting operation itself or its productivity, of the flows of molten metal within the casting tube.
This is why, for more than thirty years now, and with various but always technically probative fortunes, steel continuous casting processes always employ electromagnetic forces for the purpose of constraining these liquid metal flows to various circulatory modes, some of which, depending on the situation and the desired effects, are considered to be more appropriate than others.
Electromagnetic stirring used in this way may be performed level with the mold itself and/or level with the secondary cooling zone of the casting machine.
In the case of in-mold stirring, the magnetic field acting through the long copper walls is produced by inductors that are placed either directly submerged in the upper water chamber of the mold or in separate compartments and therefore provided with their own cooling section.
Several types of in-mold electromagnetic stirring are being practiced at the present time. They may be briefly reviewed as indicated below.
A first type (see for example JP 1 228 645 or EP 0750958) consists of a gyratory movement of the molten metal level with the meniscus around the casting axis, so as to improve the quality of the surface of the cast products. To do this, horizontally travelling magnetic fields are applied in the meniscus region over the entire width of the long sides of the mold, the direction of travel being reversed between one long side and the other. To do this, a pair of polyphase inductors, with a flat structure, of the “asynchronous linear motor stator” type, is mounted in the upper part of the mold, each inductor spreading over the entire width of the long side.
A second recommended type of stirring consists in positioning the inductors roughly at mid-height of the mold so as to be able, this time, to apply, at the outlets of the submerged nozzle, a magnetic field that travels on the half-widths of the long sides. This field is produced by flat polyphase inductors mounted facing the long sides of the mold, this time as two pairs of inductors, one pair per long side, the inductors forming a pair being placed symmetrically on either side of the casting axis defined by the nozzle and each covering approximately one half-width of the long side. The assembly formed by these four inductors is connected to one or more polyphase power supplies which provides coherent control of the whole assembly. Thus, the magnetic field produced travels in opposite directions along the two inductors from one pair and in the same direction along the inductors from different sides facing each other on either side of the cast product.
In a first version, often referred to by the term EMLA (see for example EP 1551580), the field travels to the outside, that is to say from the nozzle towards the short sides of the mold, and therefore cocurrently with the jets of molten metal arriving in the mold via the nozzle outlets. The primary purpose in this case is to promote, or stabilize, a configuration called a “double roll” of the liquid steel flow in the mold. A “double roll” configuration proves to be favourable, in particular for uniform influx of heat in the region of the meniscus, that naturally tends to cool by heat loss during casting, despite the presence of the covering slag.
In another version, referred to by the term EMLS (see for example EP 0 550 785), the magnetic field this time travels inwards, from the short sides towards the nozzle, and therefore countercurrently with the metal jets arriving in the mold. The purpose in this case is to “brake” the jets of metal so as to moderate their intensity in order to reduce the fluctuations in meniscus level and turbulence caused by too great a flow velocity.
Of course, these various examples do not constitute an exhaustive list of the possible ways of employing in-mold electromagnetic stirring for continuous casting currently available to the metallurgist. However, they do represent the two broad classes of stirring currently recommended for casting flat products (rotation at the meniscus or assistance to the jets emanating from the nozzle by braking or accelerating) which the metallurgist is confronted with when he has to make a choice in favour of one technology and rejecting the other. At the present time each stirring technology is exclusively, or almost exclusively, dedicated to one of these two aforementioned stirring modes so that the choice of stirring equipment is constricting, as being selective of the single type of stirring that this equipment will permit, the satisfactory operating conditions in all cases.